Exploring, drilling and completing hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. As a result, over the years well architecture has become more sophisticated where appropriate in order to help enhance access to underground hydrocarbon reserves. For example, as opposed to wells of limited depth, it is not uncommon to find hydrocarbon wells exceeding 30,000 feet in depth. Furthermore, as opposed to remaining entirely vertical, today's hydrocarbon wells often include deviated or horizontal sections aimed at targeting particular underground reserves.
While such well depths and architecture may increase the likelihood of accessing underground hydrocarbons, other challenges are presented in terms of well management and the maximization of hydrocarbon recovery from such wells. For example, during the life of a well, a variety of well access applications may be performed within the well with a host of different tools or measurement devices. However, providing downhole access to wells of such challenging architecture may require more than simply dropping a wireline into the well with the applicable tool located at the end thereof. Thus, coiled tubing is frequently employed to provide access to wells of such challenging architecture.
Coiled tubing operations are particularly adept at providing access to highly deviated or tortuous wells where gravity alone fails to provide access to all regions of the wells. During a coiled tubing operation, a spool of pipe (i.e., a coiled tubing) with a downhole tool at the end thereof is slowly straightened and forcibly pushed into the well. This may be achieved by running coiled tubing from the spool and through a gooseneck guide arm and injector which are positioned over the well at the oilfield. In this manner, forces necessary to drive the coiled tubing through the deviated well may be employed, thereby delivering the tool to a desired downhole location.
As the coiled tubing is driven into the well as described, a degree of fluid pressure may be provided within the coiled tubing. At a minimum, this pressure may be enough to ensure that the coiled tubing maintains integrity and does not collapse. However, in many cases, the downhole application and tool may require pressurization that substantially exceeds the amount of pressure required to merely ensure coiled tubing integrity. As a result, measures may be taken to prevent fluid leakage from the coiled tubing and into the well. As described below, the importance of these measures may increase as the disparity between the high pressure in the coiled tubing and that of the surrounding well environment also increases.
For example, it would not be uncommon for a low pressure well of about 2,000 PSI or so to accommodate coiled tubing at a depth of about 10,000 feet. Due to the depth, if the coiled tubing is filled with a fluid such as water, hydrostatic pressure exceeding about 4,350 PSI would be found at the terminal end of the coiled tubing. That is, even without any added pressurization, the column of water within the coiled tubing will display pressure at the end of the coiled tubing that exceeds the surrounding pressure of the well by over 2,000 PSI. Therefore, in order to prevent uncontrolled leakage of fluid into the well from the coiled tubing, a backpressure valve may be located at the terminal end of the coiled tubing. In this manner, uncontrolled leakage may be avoided, for example, to avoid collapse of the coiled tubing as noted above, to allow for effective pulse telemetry through the coiled tubing, and for a host of other purposes.
In many circumstances, downhole tools may be provided downhole of the backpressure valve. For example, a clean out tool configured for washing out debris within the well may be coupled to the backpressure valve. For such an application, pressure may be actively provided through the coiled tubing from surface equipment at the oilfield. As such, the backpressure valve may be remotely controlled so as to allow a controlled flow of pressurized fluid through to the clean out tool for the application.
Unlike the above-noted clean out tool however, certain downhole tools require the use of a ballistic actuator such as a spherical ball, dart, or other mechanical projectile which is dropped into the coiled tubing at the surface of the oilfield. In these applications, the ballistic actuator may make its way downhole in accordance with any fluid flow through the coiled tubing with the purpose of reaching and mechanically activating a firing head of the downhole tool. For example, downhole perforating guns are often fired by this technique. Thus, rather than rely on fluid flow and pressurization to activate a perforating gun, the described ballistic actuator is dropped through the coiled tubing line with the purpose of reaching a firing head of the gun to mechanically effect its firing into the wall of the well.
Unfortunately, as detailed above, a backpressure valve may be disposed between the coiled tubing and the downhole tool. As indicated, this may not be of particular concern where the downhole tool is a hydraulic clean out tool. However, for a downhole tool that requires activation by a ballistic actuator, such as the above noted perforating gun, this is not the case. That is, the presence of a backpressure valve at the end of the coiled tubing prevents the ballistic actuator from reaching the perforating gun. As a result, downhole tools actuated by a ballistic actuator may be avoided where coiled tubing that includes a backpressure valve at its terminal end is employed. Thus, as a practical matter, where a pressure differential between the well and coiled tubing is significant enough to require use of a backpressure valve, ballistically actuated downhole tools may not be effectively employed in the operation.